One image-forming process using silver halide photographic materials is an image-forming process using the so-called scanner system, i.e., a process of scanning an original and exposing a silver halide photographic material in conformity with the image signal to form a negative image or a positive image corresponding to the original image. Various recording devices for the image-forming process using the scanner system are known and a glow lamp, a xenon lamp, a mercury lamp, a tungsten lamp, a luminous diode, etc., have been used as a recording light source for these recording devices. However, these light sources have the practical disadvantage that the output is weak and a short life. A scanner using a coherent laser light source such as Ne-Ne laser, argon laser, He-Cd laser, etc., as the light source for the scanner system is a system for overcoming these difficulties. However, such a system has also the disadvantages that the apparatus is large and expensive, a modulator is required, and further a safe light for a photographic material is restricted since visible light is used. This makes the handling of photographic materials troublesome.
On the other hand, semiconductor laser has the advantages that the apparatus for the laser is small and inexpensive, the laser can be easily modulated, the life is longer than foregoing lasers, and also since a semi-conductor laser emits in an infrared region, a photographic material with light sensitivity in an infrared region can be used, whereby a bright safe light can be used to facilitate the handling of the photographic light-sensitive material. However, a photographic material with high sensitivity in an infrared region and excellent shelf life has not yet been developed and hence the characteristics of semiconductor laser with the foregoing excellent advantages cannot be fully utilized.
A commercially available photographic film with a sensitized infrared spectral region is, for example, HIE 135-20 made by Eastman Kodak Company but the manufacturer recommends the photographic material be stored in a cold or refrigerated state. As will be understood from these recommendations, a photographic material having sensitized infrared region has unstable sensitivity and a specific care is necessary for storage.
As a production technique for photographic light-sensitive materials, it is known to employ a spectral sensitization technique, that is, a technique of expanding the light-sensitive wave length region of a silver halide photographic emulsion to a long wave length side by incorporating certain cyanine dyes in the silver halide emulsion and also to employ this technique for sensitizing not only to light in a visible region but also in an infrared region. For spectral sensitization in an infrared region of a photographic material, sensitizing dyes having absorptions to infrared light are used and they are described in, for example, Mees; The Theory of the Photographic Process, 3rd Edition, pages 198-201, MacMillan Co., (1966).
In this case, it is desired for the photographic material thus sensitized to have a high spectral sensitivity, that is, a high sensitivity for infrared light and a sensitivity of the photographic material which changes least during storage. Many sensitizing dyes have been developed for this purpose. For example, sensitizing dyes are described in U.S. Pat. Nos. 2,095,854; 2,095,856; 2,955,939; 3,482,978; 3,552,974; 3,573,921; 3,582,344, etc. However, even using these sensitizing dyes, the sensitivity and the shelf life of photographic materials are still unsatisfactory.
On the other hand, it is known that the spectral sensitivity in photographic materials is greatly increased by adding certain specific organic compounds in addition to the spectral sensitizing dye and this effect is known as a super dye sensitization effect. Since the addition of an organic compound or an inorganic compound generally does not increase the sensitivity of a photographic material or decreases the sensitivity, the super dye sensitization effect is said to be a specific phenomenon but a very severe selectivity is required for the sensitizing dye and the organic or inorganic compound used for the combination. Therefore, even a slight difference in the chemical structure of the compound markedly influences the super sensitizing action and hence it is difficult to select combinations for the super sensitization from a simple estimation of suitable compounds from the chemical structures thereof. Triazine derivatives described in, for example, U.S. Pat. Nos. 2,875,058 and 3,695,888, mercapto compounds described in U.S. Pat. No. 3,457,078, thiourea compounds described in U.S. Pat. No. 3,458,318, pyrimidine derivatives described in U.S. Pat. No. 3,615,623, etc. are known as organic compounds used for super sensitization with sensitizing dyes. Also, U.S. Pat. No. 4,011,083 discloses that an infrared sensitization is achieved by using an azaindene compound and a desensitizing amount of an infrared sensitizing dye.
The techniques described in the above patents certainly increase the infrared sensitivity and improve shelf life to some extent but the effects are still unsatisfactory. Thus, super sensitizing means for further increasing infrared sensitivity and shelf life is desired.
On the other hand, when a silver halide emulsion is in a liquid state before coating, the sensitivity and fog thereof tends to change. In particular the change in sensitivity and fog caused by desorption and decomposition of sensitizing dyes tends to occur. The change in photographic characters of a silver halide emulsion in a liquid state before coating is a large problem in production of photographic materials. However, stabilizers such as 1-phenyl-5-mercaptotetrazole, etc., ordinarily used as stabiliers for silver halide photographic emulsions are ineffective to improve the stability of silver halide photographic emulsions containing infrared sensitizing dyes, which are sol states before coating. Therefore, development of a technique for specifically improving the stability of silver halide emulsions containing infrared sensitizing dyes which are in the liquid states has been desired.